This invention relates to a method for permanently increasing the laser induced damage threshold of optical thin films.
The performance of high peak power lasers, such as those used for fusion research and materials processing, is often limited by the damage threshold of optical components that comprise the laser chain. In particular, optical thin films generally have lower damage thresholds than bulk optical materials, and therefore thin films limit the output performance of these laser systems. Optical thin films are used as high reflectors, polarizers, beam splitters and anti-reflection coatings.
The Nova project at Lawrence Livermore National Laboratory is designed to study the use of lasers to produce fusion by inertial confinement. The 1.06 .mu.m wavelength Nova laser output is limited, in part, by the damage threshold of large aperture (approximately 1 m diameter) dielectric thin films coated on flat substrates. Proposed future fusion lasers require optical coatings with laser induced damage thresholds that exceed a fluence of 35 J/cm .sup.2 in 10 ns pulses at the 1.06 .mu.m wavelength. Fluence is defined in the specification and claims for a pulsed laser of a specified wavelength and specified pulse length as the energy per unit area delivered by a single pulse. Prior to the invention, the highest damage thresholds were in the range from 10-20 J/cm.sup.2 in a 10 ns pulse at the 1.06 .mu.m wavelength. Therefore, a method of increasing the laser damage threshold of dielectric optical thin films (or coatings) is needed.
Several researchers have previously reported that the damage thresholds of some optical materials could be increased by first illuminating the optical materials with sub-threshold fluences of laser light. Some such examples are: Swain, J. E.,Lowdermilk, W. H., Milam, D, "Raising the Surface Damage Threshold of Neutral Solution Processed BK-7 by Pulse Laser Irradiation," Nat. Bur. Stand. (US) Spec. Pub. 669, 1982 November 292 p.; Frink, M.E., Arenberg, J. W., Mordaunt, D. W., Seitel, S. C., Babb, M. T., Teppo E. A., "Temporary Laser Damage Threshold Enhancement By Laser Conditioning of Antireflection-Coated Glass," Appl. Phys. Lett. 51, 1987, 415 p.; Arenberg, J. W., Mordaunt, D. W., "Experimental Investigation on the Role of Wavelength in the Laser Conditioning Effect," Nat. Inst. Stand. & Tech. (US) Spec. Pub 756, 1987 October 369 p.; Wilder, J. G., Thomas, I. M., "Effect of n on 1 Laser Treatment on Damage Threshold of Selected Optical Coatings," Nat. Inst. Stand. & Tech. (US) Spec. Pub. 775, 1988 October, 259 p.; Stewart, A. F., Guenther, A. H., Domann, F. E., "The Properties of Laser Annealed Dielectric Films," Nat. Inst. Stand. & Tech. (US) Spec. Pub. 756, 1987 October, 369 p.; and Swain, J. E., Stokowski, S. E., Milam, D., Kennedy, G., "The Effect of Baking and Pulsed Laser Irradiation on the Bulk Laser Damage Threshold of Potassium Dihydrogen Phosphate Crystals," Appl. Phys. Lett. 41 (1982) 12. Increases in damage threshold of a factor of three have been reported. This laser conditioning effect has not been put to practical use however, for several reasons:
a) Early studies indicated that laser conditioning has only a temporary effect; damage thresholds could only be improved for a few hours or days. PA1 b) The relative importance of film parameters (such as design, materials and deposition method) on conditioning was not known. PA1 c) The laser conditioning was sometimes achieved using illumination at wavelengths different than the wavelength that the optics were intended for. Therefore, an additional laser system was required for conditioning. PA1 d) Methods for conditioning large aperture optics had not been addressed.